difficult to assess over the internet – whats the condition of the rest of the car.
unless you can fit the engine your self and have the tools i wouldnt have thought it would be economical on an 04 astra.
which engine is it – is it one with a flaw – i had a golf with major issues down to the materials the block was constructed from- and it was most likely a new engine would have the same issues …. i got rid..
also have had bad experiances with having to reprogram immobilisers on Vauxhall cars after just changing fuel pumps – would imagine it wouldnt like an engine swap much – although perhaps thats because mines was a bastardized isuzu motor….
I would stick it on gumtree and local facebook groups – always a mechanic out tere looking to turn a buck and will pay above scrap for a car with a decent length test on.
an engine swap doesnt take long if you know what your doing.
ps theres not many ways for white smoke (water) to be coming out the exhaust and for the water to disapear from the header…..how ever it is always worth getting a second opinion.